[SalesForce] Apex String Split with single quotes

It depends what you mean by "such that each of its characters can be manipulated". Apex Strings (as in Java) are immutable, so a true manipulation of the source value is not going to happen. Nor is there a char datatype in Apex. You can inspect individual "chars" by using the String.mid method, or you can efficiently create an array of 1-char long Strings from your source string (similar to a char[]). You could then write new "chars" into elements of that array, and the join/recombine the parts as necessary, to form the desired result in a new String. Example:

// splitting on empty gives you an array of the string's "chars":
String source = 'foo bar';
String[] chars = source.split('');
// the 1st element in an Apex '' split is garbage; remove it:
chars.remove(0);
System.debug(chars);
// change a char:
chars[6] = 'z';
// prints "foo baz":
System.debug(String.join(chars, ''));

The format method automatically does the commas and periods or visa versa (depending on whether your profile is in the US or elsewhere)

Assuming the $ sign applies to all cases in your org you can simply use

Decimal input = 2000;
String output = '$' + String.valueOf(input.format());

Note the String.valueOf(). This is required because the input variable is defined as a Decimal.

Edit: I noticed a bug with the Decimal.format() method where it will not show any numbers past the decimal point if there are only zeros there, ex. $100.00. To solve this I came up with this method.

private String getCents(Decimal x){
    String y = String.valueOf(x);
    String z = '.';
    if(y.contains(',')) z = ',';
    y = y.substring(0, y.indexOf(z));
    if(x - Decimal.valueOf(y) == 0)
        return String.valueOf(x.format()) + z + '00';
    else return String.valueOf(x.format());
}

Then to update the example above, it would be:

Decimal input = 2000;
String output = '$' + getCents(input);